The Venezuelan president also warned that he would halt oil supplies to the US if it attacked Venezuela.

Jennifer Rahimi, a US embassy spokeswoman in Caracas, said "we're investigating, but we haven't seen anything official".

Plot allegation

Chavez announced the decision during a televised speech, hours after saying his government had detained a group of alleged conspirators in a plot to overthrow him.

"The ambassador of the United States is conspiring against democracy and wants Bolivia to break apart"

Evo Morales,
Bolivia's president

The Venezuelan president accused the group of current and former military officers of trying to assassinate him with backing from the US.

Al Jazeera's Mariana Sanchez reported from Caracas that Chavez critics were saying that he was trying to create instability in an attempt to shore up what they said was his plummeting popularity ahead of elections.

Morales expelled Phillip Goldberg, the US ambassador, from La Paz on Wednesday, accusing him of instigating violent protests in Bolivia.

On Thursday, at least 10 people were killed in the Andean nation during continuing protests, led by rebel state governors demanding greater autonomy and opposing Morales' plans to give more land to the poor.

"The ambassador of the United States is conspiring against democracy and wants Bolivia to break apart," said Morales, who is a fierce critic of US policy.

The US responded on Thursday, announcing that it had expelled the Bolivian ambassador.

Sean McCormack, a state department spokesman, said the US had "officially informed the government of Bolivia of our decision to declare Ambassador Gustavo Guzman persona non grata".

Russian bombers

Tensions between the US and Venezuela had already risen after Russia sent two strategic bombers to Venezuela for military exercises.

Chavez said the aircraft had arrived to counter US influence in the region.

Russia has sent two strategic bombers to
Venezuela [EPA]
On Thursday, the US said it was monitoring the situation.

"It is something that we will watch very closely, as we have with the movements of other military assets for the stated purpose of this joint exercise," McCormack said.

The Russian move is reported to be Moscow's first strategic deployment in the Western hemisphere since the end of the Cold War and comes amid increased tensions between Russia and the US over the recent conflict with Georgia.

Moscow remains angry that US used military vessels to carry aid to Georgia during Russia's intervention there.

However, Russia has said that the Venezuela deployment is part of a training exercise and that the bombers will return to Russia within days.